1. Field of the Invention
This invention relates to ceramic fibers useful for insulation and reinforcement purposes and more particularly relates to nonoxide fibers suitable for these uses.
2. History of the Prior Art
Ceramic fibers have been used for an extended period of time in the prior art for various purposes including insulation and, to some extent, reinforcement. An example of such a fiber that might be considered ceramic is the naturally occurring asbestos fiber which has now fallen into disfavor because of health problems. In addition, for long periods of time, alumina, silica, and alumina silicate fibers have been manufactured for various uses, including reinforcement, insulation and as fillers. Such oxygen containing fibers do not, however, have the properties desired for certain high performance applications. In particular, such oxide type fibers often do not have coefficients of thermal expansion which are as close as desirable to the matrix material which is being reinforced and usually do not have heat resistance as high as desirable for very high temperature performance applications. In addition, corrosion resistance of oxide fibers is often not as good as desirable in certain environments such as molten metals. Furthermore, many oxides do not have an elastic modulus as high as desirable for stiffening in reinforcement applications. Examples of such oxide type fibers may be found in U.S. Pat. Nos. 4,071,594; 4,250,131; 3,808,015 and 3,992,498.
Such oxide fibers in the prior art have generally been manufactured by melt spinning or blowing or by drawing of a precursor material, sometimes with included oxide powders, which is then often converted to an alumina material. Such methods have not generally been found applicable, to this point, to very high temperature nonoxide ceramics. Such very high temperature nonoxide ceramics tend not to melt at manageable temperatures and in fact tend to decompose or sublime prior to melting. Attempts have also been made to make nonoxide ceramic fibers by means of precursors, e.g. as taught in U.S. Pat. Nos. 3,529,044; 4,117,057 and 4,158,687. Such fibers have not, however, been as good as desirable since such precursors tend to leave large amounts of substances, e.g. oxygen, in the fibers which are often detrimental to their properties often reducing strength, temperature resistance and chemical resistance.
Attempts have, nevertheless, been made to utilize nonoxide ceramic materials for high temperature insulation or for high performance reinforcement. An example of such a reinforcing material is silicon carbide whiskers which have been considered by some to fall within the general description of a fiber. Such whiskers are, in fact, believed to be single elongated crystals which are difficult to manufacture, even by batch methods, and are very costly. In addition, the diameters and lengths of such whiskers which can be manufactured are exceedingly restricted. Other attempts to manufacture nonoxide ceramic fibers have not usually been highly successful since materials are generally incorporated into the fiber which do not have high performance characteristics.
There is therefore a need for a high performance nonoxide ceramic fiber of high purity which can be manufactured, desirably continuously, to various specifications including cross sectional shape and length. It would be desirable to have such a fiber which is stable, both chemically and physically, in an inert atmosphere at temperatures of 1700.degree. C. and higher and stable even in air at temperatures of 1500.degree. C. and higher.
Furthermore, ceramic fibers manufactured in the prior art are typically difficult to handle, especially when it is desired to utilize such fibers for formation of textile fabrics. It is therefore desirable to have a method and a product which will permit woven products to be developed which contain nonoxide ceramic fibers.
A particularly interesting prior art patent is U.S. Pat. No. 4,559,191 which describes forming a fiber by hydrostatically pressing a ceramic powder as a core in a hollow polymer fiber. The disadvantages of such a process is clear, i.e. difficulty in getting the powder into the core, hydrostatic processing requirement and expense. In addition, the fiber would not be expected to have a high cross sectional aspect ratio to obtain improved bending resistance in the direction of the long cross sectional axis. Additionally, the fiber of this U.S. Pat. No. 4,559,191 would not be expected to have a smooth surface, which is desirable to reduce defects. This is true because it is generally known that isopressed sintered articles have rougher surfaces than drawn or extruded sintered articles. "Smooth" as used herein therefore means smoother than the surface of an isopressed article made of the same material and shape. Such "smooth" articles, when large enough, generally are characterized by a surface which does not scatter light and thus reflect light brightly, i.e. the surface shows gloss and usually will reflect images. The "rough" surface of a sintered isopressed article, on the other hand, tends to have a dull finish resulting from light scatter. "Smooth" is, however, not intended to exclude cross sections which are not circular or eliptical. Surfaces which have a relatively uniform cross section along their longitudinal axis can reflect light, uniformly from their surfaces, e.g. starshaped cross sections.